It depends on what facet of the mainstream market we are talking about. Pretty much any distro will work for the higher end of the mainstream these days(No CS degrees or serious formal training; but know a thing or two and are willing to mess with it a bit). Hardware support, especially in laptops, can still be a problem; but less than it was.
The bottom of the mainstream market is underpenetrated by Linux; but could actually be very well served by it. These are the users who buy a complete setup from Dell and just want to keep checking their email, looking at web pages, IMing, and possibly some light gaming, preferably as long as possible. They don't really install new software or hardware, and when they do they tend to buy cautiously. Obviously, this market would have a huge amount of trouble setting up Linux(not that they could install Windows, either) but they wouldn't have to. The people they buy the box from would install it, make sure the drivers work, add the basic suite of tools and games, etc. From this point on, Linux would actually be better, as it is much less likely than Windows to get horribly screwed up by viruses and spyware and general weirdness(especially if the default configuration was set to update itself from time to time, via Apt cronjob or similar). Here the distro wouldn't much matter. The manufacturer would probably just buy a mess of Redhat or Suse licences, or maybe hire a few in-house guys to knock together a branded version of Debian, or Gentoo or something. Which distro wouldn't much matter to the user; because they would never have to touch it. I'm actually rather suprised that this sort of thing isn't more common.
The real trouble spot for Linux in the mainstream is the middle of the market. The guy who sort of knows what he is doing; but doesn't have a geek's patience for mucking about with drivers. The one who is savvy enough to buy a shiney new video card; but not savvy enough to deal with getting it running under Linux. The one who buys peripherals from a variety of sources, which may or may not have driver support. These people are probably better served by Windows, which takes a touch of knowledge to secure; but is often easier to work with if you don't want to do unusual stuff and do know how to avoid getting infected by the usual internet malware.